1. Field of the Invention
This invention relates to an attachment lens, and in particular, to a small-sized attachment lens which serves as a passage not only for the rays used for photography but also various other rays, and to a camera system equipped with such an attachment lens.
2. Description of the Related Art
Heretofore, it has been a well-known practice in small-sized cameras to arrange, in the vicinity of the optical path for the photographic lens, an exterior-finder optical system and a photometric optical metering system. This type of arrangement is advantageous also in an electronic still camera, since, by converting the conventionally used single-lens reflex type into this arrangement, further miniaturization in camera size can be realized It is to be noted in this regard that the nearer the optical axis of each optical system is situated to that of the photographic lens, the more effectively can any parallax be corrected and any vignetting removed.
Ideally, a system in which the photography rays, the viewfinding rays and the photometry rays (and further, in the case of an electronic still camera, the colorimetry rays) are arranged on the same axis, i.e., the TTL system, provides the maximum space efficiency and is most preferable in terms of optical characteristics. However, this is only true with the space efficiency in the camera front section. In the rear section of the camera, the system can involve an even more complicated mechanism and constitute an obstacle to miniaturization, since in the case of the TTL system, it is often necessary for the bundles of rays passing through the photographic lens to branch off according to the purpose for which the camera is used.
In view of this, a non-TTL type arrangement is generally adopted for the exterior-finder optical system, and the exterior-photometric optical system, for example. This is especially the case with inexpensive cameras, overall miniaturization being effected by miniaturizing the individual parts and designing the various sensors as integrated components.
The operation of attaching an attachment lens to a camera system involves the following problems:
The rays useful for photography, viewfinding, photometry and colorimetry diverge as they travel away from the camera front and approach the subject, with the result that the areas through which they pass overlap. As areas farther from the camera are considered, this naturally results in areas through which these rays overlap practically completely.
This is also true with the attachment lens, which is arranged at a position situated somewhat apart from the camera front. That is, the nearer to the subject the attachment lens is situated, it is more likely that the respective passage areas for the different rays of light will overlap.
In the case of a wide angle converter lens, used for widening the view angle of a photography lens, at least two groups of lenses are needed for its construction. One of these groups, situated nearer to the subject, exhibits negative refractive power, and the other exhibits a positive refractive power. Likewise, at least two groups of lenses are needed for the finder optical system i order that it can be used for correcting the view angle of a converted photography lens. This will be referred to as the finder-correction optical system of an attachment lens. As stated above, miniaturization in camera size will result in the lenses in the correction optical system or the lens barrel section frequently interrupting the rays for photography. Conversely, the lens barrel of the photography lens will interrupt the rays of the finder-correction optical system and the photometric optical system, for example.
This problem is attributable to the fact that the individual lenses are independent of each other in terms of function, so that a large amount of space is needed to accommodate them.